This invention relates to radio direction finding systems adapted for homing operations and more particularly to a unique antenna array for use with phase front homing systems.
Radio direction finding systems enabling aviators to navigate aircraft towards distant beacon transmitters may operate by simply indicating only the sense of direction towards the beacon, i.e. right or left without any indication of the actual angle in degrees. Such simple systems are commonly called homing systems. The pilot observes an indicator on the homing apparatus, such as a needle, and maneuvers his aircraft so as to center the needle and thus "home" on the distant beacon transmitter. It is desirable that such homing systems should function accurately over wide frequency ranges. In the military VHF band, for example, it is desirable that the homing system extend over a relatively wide band extending from 30 to 76 MHz. Usually, however, most conventional homing systems will operate at a limited number of frequencies. Poor broadband performance of conventional homing systems is attributed to such factors as limited dynamic range and AGC characteristics of radios and, more significantly, the failure of the antenna system to provide symmetrical radiation patterns at all frequencies. To alleviate most of the problems of such conventional homing systems, so called phase front homing systems have been devised. Basically, the phase front homing system derives the desired sense of the direction to the beacon transmitter by utilizing directly the phase difference between the signals picked up by two antenna elements. In this sense, greater system accuracy is realized because the phase shift between antennas, ideally, is unaffected by amplitude variations. In contrast, the conventional homing system derives its information from both phase and amplitude characteristics of the antenna array and therefore it is more prone to inaccurate performance. Such phase front homing systems usually employ conventional separate, visible add-on antenna elements such as whips or dipoles. However, it is well known that the phase shift between such separate antenna elements of the homing antenna array is greatly affected by such factors as the type of aircraft and the location, orientation, and symmetry of the array.